The use of acidizing systems based on acid precursors has become widespread in oilfield chemical treatments over the last decade or so. Producing organic acid in-situ from acid precursors, rather than using an organic or mineral acid directly, can deliver excellent zonal coverage. In addition the use of acid precursors offers considerable health, safety and environmental advantages.
Treatment formulations based on acid precursors are generally designed to deliver a certain amount of acid from the treatment fluid within a desired timescale at the prevailing temperature. The acid produced from the acid precursor is available to solubilise at least a portion of the targeted acid-soluble materials, for example carbonate rock, carbonate scale or carbonate components of drill-in fluid filter cakes, so that they can be removed from the treated zone in the form of dissolved salts.
The most commonly used acid precursors are carboxylic acid esters, particularly esters of formic, acetic and lactic acid. These are suited to use in different temperature ranges. They have a relatively high yield of acid, with the acid precursors and the products of acidizing all being soluble.
If using these acid precursors in the absence of an enzyme or other catalyst, the typical temperature range in which acetic and lactic acid precursors are used is from about 85° C. and preferably from about 100° C. up to about 160° C. Precursors of formic acid hydrolyse more readily than precursors of acetic or lactic acid, so can generate acid in-situ at lower temperatures. The typical temperature range in which precursors of formic acid are used is from about 30° C. to about 120° C.
The rate of acid generation from precursors of formic, acetic or lactic acid may, if desired, be increased using a suitable enzyme catalyst such as a lipase, esterase or protease enzyme (U.S. Pat. No. 5,678,632). Treatment formulations are designed to give acceptable rates of acid generation and therefore acceptable treatment (shut-in) times under the conditions of use.
Unfortunately, at temperatures lower than about 30° C. acidizing based on the most commonly used acid precursors can proceed more slowly than is generally acceptable to operators.
For example, at 27° C. it typically takes about 6 days to convert a formic acid precursor, used at 10% w/v in a treatment fluid, to formic acid. This can be reduced to about 2 days by incorporating enough of a suitable enzyme into the treatment formulation. However, this increases the cost.
There are a number of low temperature underground formations found worldwide at temperatures below about 30° C. For example some formations in Canada and Russia are at temperatures as low as 7° C. At such low temperatures, acidizing treatments based on formic acid precursors, even those containing ester-hydrolysing enzymes, would require long shut-in periods likely to be unacceptable to operators.
Another problem associated with the use of formic acid precursors in dense formate brines is the potential for precipitation of calcium salts. In such treatments, the solubility of calcium formate may be exceeded, resulting in the precipitation of calcium formate as a solid in the treatment fluid. US 2013/0213659 teaches that by including a suitable chelant or an alkyl glucoside surfactant in the treatment fluid, precipitation of calcium formate when acidizing with a formic acid precursor in dense formate brines may be avoided.
There is a continuing need for acidizing processes that are low hazard and that are based on environmentally acceptable components.
There is a specific need for in-situ acidizing processes that are effective within acceptable treatment timescales at low temperatures, particularly at temperatures in the range from about 5° C. to about 30° C., for oilfield and other applications. Furthermore, it is necessary to avoid the precipitation of solid by-products, such as acid salts, as the acidizing process takes place.